On dual detector cameras, the acquisition can take place either by tomography with perpendicular or parallel detectors that revolve around the organ, or by translation of the detectors facing each other on either side of the patient's body.
The non uniformity of the attenuation due to variable attenuation coefficients, depending on the type of tissue (bone, breast, lung, muscle, . . . ) causes artifacts which alter the precision of the emission images. The conventional attenuation correction procedures are inappropriate because they presume a constant distribution of the attenuation coefficients. An acquisition by transmission, using an external radioactive source, allows for the calculation of a table of attenuation coefficients that are specific to the patient, a table that will be used to correct this emission of images. This process allows for a Transmission Attenuation Correction, better known as "TAC".
The correction system used to implement this TAC process consists of a radioactive source that emits gamma rays of known energy that sweep the active surface of the detector through the patient's body. The attenuation of energy caused by the patient's body is measured in this detector. The result of this measuring, appropriately treated, provides data that allows for the correction of the attenuation of photon energy emitted by the irradiated organ and therefore, for the acquisition of a more precise image.
The TAC system radioactive source is a linear rod whose radiation is transmitted in the direction of the detector through a collimator; it moves parallel to itself in a plane that is parallel to the detector's surface of detection.
Over time, this radioactive source progressively loses its radiation power whereas the energy received by the detector must remain approximately constant in time. Therefore, between the source and the collimator, we have an attenuator that belongs to a series whose efficiencies are decreasing. On the other hand, when the source no longer emits rays, it must be replaced.
As it now stands in the technique, the replacement of the attenuator and of the source are delicate operations that must be performed on site with great care by specialized personnel in order not to be subject to a prolonged and therefore dangerous radiation.
The current technique represented in particular in the U.S. Pat. Nos. 5,596,197 and 5,598,003 which refer to the sweeping direction in very specific conditions does not address the problems brought up.